1. Technical Field
The present invention relates to an ink set.
2. Related Art
When achromatic gradation patches having a gradation from black to white are formed with a black ink containing only carbon black as the colorant, the hue of the gradation patch varies in such a manner that the a* and b* values turn away from the center (a* value=0, b* value=0) representing the genuine achromatic color, as shown in FIG. 1 illustrated in detail later. It is thus known that the color reproductivity of such an image is not satisfactory. One in this gradation patch, the hue lies at the center of the a*b* coordinates when the color is close to white. The color becomes slightly yellowish black as the amount of carbon black applied is increased, and when the carbon black is further applied to fill dots closely, the hue tends to return to the center of the a*b* coordinates. The term “color reproductivity” refers to a property of an ink representing small variation in hue for any color.
Ink sets have been proposed which are prevented from forming a slightly yellowish portion between colors in an achromatic image and thus achieve good color reproduction. For example, JP-A-2003-55592 discloses a black ink set including a plurality of black ink compositions, each containing a plurality of colorants: a carbon black, magenta pigment and cyan pigment. The carbon black contents are different among the black ink compositions, and the magenta pigment and cyan pigment contents in each black ink composition are adjusted so that the chroma C* of the ink composition is 3 or less.
The pale black ink, such as a gray ink, of the ink set disclosed in this patent document is prepared by adding very small amounts of two or more pigments. In such an ink composition, an error of the pigment content leads to a large variation in hue and causes variation among lots of the ink composition even if the error is very small. In addition, the color inconstancy (dependence on source light) of a black ink depends on the spectral characteristics of the carbon black in the black ink, that is, by the carbon black content in the black ink. Gray inks or other pale inks containing a carbon black with a low content have undesirable high color inconstancy.